It is known to apply a coating to bore (hole) cutting tools such as twist drills, reamers, core drills, countersinks, thread cutting tools, milling cutters etc. Drills (sometimes referred to as drill bits) in particular are coated to extend the life of the drill and to enhance cutting speed.
The materials used for coating generally have a higher hardness than the material from which the drill bit is made. For example, titanium nitride (TiN) coated tools have a surface hardness up to 3 times harder than uncoated high speed tools. The coating is also usually refractory and resistant to high temperatures.
The coatings are typically applied to the whole of the fluted end of the drill. FIG. 1 shows such a coated drill. The coating is normally applied by physical vapour deposition (PVD). TiN coated drills are easily recognised by their bright gold colour.
The coating thickness is typically about 1-4 μm. FIGS. 2 and 3 show a schematic of a typical high speed steel HSS drill, from the side and end-on respectively. The drill 10 has a shank 12 and body 14. The drill has length 16 which includes shank length 18 and body length 20. The body is fluted and includes two helical flutes 22. Separating the two flutes are two lands 24. The lands have a leading edge 26 and heel 28.
Extending from tip 30 are lips 32 (also known as cutting edges). The tip itself includes chisel edge 34. Rake face 36 extends rearwardly from lip (cutting edges) 32.
TiN coatings are applied to the entire body 14, so that tip 30, lands 24, flute 22, etc are all coated.
The chip is the name given to material that is cut away from the work piece that is being drilled. The chip is cut from the bottom of the hole by the chisel edge 34 and cutting lips 32 and is forced into the flutes 23 by contact with the rake face 36. Rotation of the flutes causes the chips to be evacuated from the hole.
As noted above, one of the most common coating materials is Titanim Nitride (TiN). TiN has high hardness, good resistance to high temperature and a low coefficient at friction. Consequently a TiN coating can improve the performance of and life-span of a drill. Cutting force measurements indicate a that TiN coating reduces the specific cutting energy by reducing friction at the drill-chip interface.
A particular advantage of TiN coated drills, especially TiN coated high-speed steel (HSS) drills is that they can be operated at high cutting speeds (e.g. 40%-50% higher than uncoated drills) and feed rates. Furthermore, they can be used to drill holes in hard, tough and abrasive materials.
HSS drills are highly alloyed steel and common types include either molybdenum or tungsten components together with chromium, vanadium and cobalt.
A widespread use for TiN coated high speed drills (HSS-TiN) is to drill holes in titanium and titanium alloys. These materials are used, e.g. in the aerospace industry because of their good mechanical properties. For the same reasons it is difficult to drill into titanium and its alloys.
Whilst HSS-TiN drills are routinely used to drill into titanium in many industries, they have not been adopted by the aerospace industry because there is concern that TiN particles from the coated drill could transfer to the work piece during drilling. Specifically, there is concern that under drilling conditions (high temperature and high friction) particles of TiN could become impregnated in the walls of the hole being drilled and that these particles could later be detrimental to the structural properties of the work piece, for example causing cracks.
Whilst the mechanism by which small particles of TiN might weaken titanium or titanium alloy work pieces is not known, it is thought that the particles could act as stress raisers or propagate failure fractures.
Even particles as small as 2 μm may affect the performance of the work piece.
Despite the concerns of the aerospace industry, there have been no published studies that prove that TiN particles are transferred to the work piece.